The present invention relates in general to a method for forming non-woven fabrics, and, more particularly, to an improved method for improving the throughput capacity of a sifting type former employing a plurality of rotors mounted for rotation in a horizontal plane immediately above a sifting screen with each rotor being mounted for rotation about a vertical axis.
Conventionally, materials suitable for use as disposable tissue and towel products have been formed on papermaking equipment by water-laying a wood pulp fibrous sheet. Conceptionally, such equipment has been designed so that the configuration of the resulting sheet approaches a planar structure. This allows continuous operation at high speeds; and, such sheets may be formed at speeds of 3,000 to 4,000 feet per minute. Indeed, recent developments have allowed sustained production at speeds of up to 5,000 feet per minute.
Following formation of the sheet, the water is removed either by drying or by a combination of pressing and drying. As water is removed during formation, surface tension forces of very great magnitude develop which press the fibers into contact with one another, resulting in overall hydrogen bonding at substantially all fiber intersections; and a thin, essentially planar sheet is formed. It is the hydrogen bonds between fibers which provide sheet strength and, such bonds are produced even in the absence of extensive additional pressing. Due to this overall bonding phenomenon, cellulosic sheets prepared by water-laid methods inherently possess very unfavorable tactile properties (e.g., harshness, stiffness, low bulk, and poor overall softness) and, additionally, possess poor absorbency characteristics rendering such sheets generally unsuitable for use as sanitary wipes, bath and facial tissues, and toweling.
To improve these unfavorable properties, Sanford et al. U.S. Pat. No. 3,301,246 proposes improving the tactile properties of water-laid sheets by thermally predrying a sheet to a fiber consistency substantially in excess of that normally applied to the dryer surface of a paper machine and then imprinting the partially dried sheet with a knuckle pattern of an imprinting fabric. The sheet is thereafter dried without disturbing the imprinted knuckle-pattern bonds. While this method may somewhat improve the softness, bulk and absorbency of the resulting sheet, the spaces between the knuckle bonds are still appreciably compacted by the surface-tension forces developed during water removal, and considerable fiber bonding occurs. Creping is still essential in order to realize the maximum advantage of the proposed process; and, for many uses, two plies are still necessary.
As will be apparent from the foregoing discussion, conventional paper-making methods utilizing water are geared towards the high speed formation of essentially planar sheets; yet, such methods inherently possess the inefficient attribute of initial "overbonding", which then necessitates a creping step to partially "debond" the sheet to enhance the tactile properties. Also, the extreme water requirements limit the locations where paper-making operations may be carried out. Such operations require removing a large quantity of the water used as the carrier, and the used process water can create an associated water pollution problem. Still further, the essential drying procedures consume tremendous amounts of energy.
Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been used for applications where either little strength is required, such as for absorbent products--i.e., pads--or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant--i.e., various specialty papers. U.S. Pat. No. 2,447,161 to Coghill, U.S. Pat. No. 2,810,940 to Mills, and British Pat. No. 1,088,991 illustrate various air-forming techniques for such applications.
In the late 1940's and early 1950's, work by James D'A. Clark resulted in the issuance of a series of patents directed to systems employing rotor blades mounted within a cylindrical fiber "disintegrating and dispersing chamber" wherein air-suspended fibers were fed to the chamber and discharged from the chamber through a screen onto a forming wire--viz., J. D'A. Clark U.S. Pat. Nos. 2,748,429, 2,751,633 and 2,931,076. However, Clark and his associates encountered serious problems with these types of forming systems as a result of disintegration of the fibers by mechanical coaction of the rotor blades with the chamber wall and/or the screen mounted therein which caused fibers to be "rolled and formed into balls or rice which resist separation"--a phenomenon more commonly referred to today as "pilling". These problems, inter alia, and proposed solutions thereto, are described in, for example: J. D'A. Clark U.S. Pat. No. 2,827,668, J. D'A. Clark et al. U.S. Pat. Nos. 2,714,749, and 2,720,005; Anderson U.S. Pat. No. 2,738,556; and, Anderson et al. U.S. Pat. No. 2,738,557. However, prior to the advent of the present invention, it is not believed that systems of the type disclosed by J. D'A. Clark and his associates which employed cylindrical fiber disintegrating and dispersing mechanisms with and/or without rotors, have been suitable for use in production type, air-laid, dry fiber, web forming systems, principally because problems of pilling have not been resolved, and because of severe fiber damage due to the disintegrating action of the rotor in Clark's cylindrical chamber.
It should be noted that the aforesaid Clark et al. U.S. Pat. No. 2,720,005 discloses an air scrabbler system having a foraminous separating wall wherein slots may be formed in the wall rather than relatively small openings such as are employed with conventional woven square-mesh screens. The Clark et al. patent is silent as to the orientation of the slots. However, in the aforesaid Clark U.S. Pat. No. 2,748,429 which also contemplates the use of a slotted separating wall, the slots are shown and described as "circumferentially extending laterally spaced slots" (See, Col. 3, lines 22-23). Such slot orientation has been found to be substantially inoperable when utilizing 2-dimensional formers of the type employing a horizontally disposed rotor assembly.
A second type of system for forming air-laid webs of dry cellulosic fibers which has found limited commercial use has been developed by Karl Kristian Kobs Kroyer and his associates as a result of work performed in Denmark. Certain of these systems are described in: Kroyer U.S. Pat. Nos. 3,575,749 and 4,014,635; Rasmussen U.S. Pat. Nos. 3,581,706 and 3,669,778; Rasmussen et al. U.S. Pat. No. 3,769,115; Attwood et al. U.S. Pat. No. 3,976,412; Tapp U.S. Pat. No. 4,060,360; and, Hicklin et al. U.S. Pat. No. 4,074,393. In general, these systems employ a fiber sifting chamber or head having a planar sifting screen which is mounted over a forming wire. Fibers are fed into the sifting chamber where they are mechanically agitated by means of a plurality of mechanically driven rotors mounted for rotation about vertical axes. Each rotor has an array of symmetrical blades which rotate in close proximity to the surface of the sifting screen. The systems described in the aforesaid Kroyer and related patents generally employ two, three, or more side-by-side rotors mounted in a suitable forming head.
In an effort to overcome the productivity problems associated with such systems, complex production systems have been devised utilizing multiple forming heads--for example, up to eight separate spaced forming heads associated with multiple hammermills and each employing two or three side-by-side rotors. The most recent sifting type systems employing on the order of eighteen, twenty or more rotors per forming head, still require up to three separate forming heads in order to operate at satisfactory production speeds--that is, the systems employ up to fifty-four to sixty, or more, separate rotors with all of the attendant complex drive systems, feed arrangements, recycling equipment and hammermill equipment.